In general, an automatic transmission mounted in a vehicle includes a plurality of planetary gear sets (planetary gear mechanisms) and a plurality of hydraulic frictional engagement elements such as clutches and brakes, and is configured to provide a plurality of forward gear and usually one reverse gear by selectively engaging these frictional engagement elements by hydraulic control, and thereby switching power transmission paths through the planetary gear sets.
Recently, there have been a growing demand for increasing the number of forward gears in order to improve engine fuel consumption performance or transmission performance. For example, an automatic transmission has been proposed which includes three planetary gear sets and six frictional engagement elements, and provides eight forward gears by engaging two of the frictional engagement elements in combination.
Unfortunately, in this configuration of the automatic transmission, there are four frictional engagement elements that are disengaged in each gear. Therefore, sliding resistance between friction plates of frictional engagement elements that are disengaged, viscous resistance of lubricant oil between the friction plates, or other factors may increase driving loss in the overall automatic transmission, resulting in a deterioration of improvement effect of fuel consumption performance by multiple gears.
Patent Document 1 discloses, as illustrated in FIG. 18, an automatic transmission including four planetary gear sets PGa, PGb, PGc, and PGd, and five frictional engagement elements, namely, first, second, and third clutches CLa, CLb, and CLc, and first and second brakes BRa and BRb, and producing eight forward gears by selectively engaging three of the five frictional engagement elements. According to this transmission, the number of frictional engagement elements that are disengaged is two in each gear. This reduces the driving loss as described above.
In the automatic transmission disclosed in Patent Document 1, of the four planetary gear sets PGa, PGb, PGc, and PGd, the two planetary gear sets PGa and PGb are disposed closer to an input side such that one of the two planetary gear sets PGa and PGb is disposed radially inward from the other. This reduces the length of the automatic transmission in the axial direction.